Power transistors can be applied to digital circuits and analog circuits, and hence are widely applicable power components. Power transistors can be categorized into horizontal and vertical power transistors based on the current flow path. Common vertical power transistors include trench gate metal-oxide-semiconductor field-effect transistors (trench gate MOSFETs or UMOSFETs), V-groove MOSFETs (VMOSFETs), and vertical double-diffused MOSFETs (VDMOSFETs). Further, since the U-shaped trench of trench gate MOSFETs can effectively reduce the on-state resistance and improve the edge termination characteristics of transistors, trench gate MOSFETs may serve as promising high-frequency low-voltage power components.
However, conventional trench power transistors such as trench gate MOSFETs, when operated in the blocking mode, are required to support all the forward blocking voltage across the drift region (e.g. an N-type drift layer). Therefore, in order to support a higher forward blocking voltage, it is necessary to reduce the doping concentration in the drift region. As a trade-off for such reduction, the on-state resistance is increased, which is unfavorable to the component characteristics. In addition, since the gate electrode in the trench of conventional trench power transistors is unsatisfactorily large in volume, the gate-to-drain capacitance (CGD) is unsatisfactorily large, and the operation of conventional trench power transistors at high frequency is limited.
Referring to FIG. 1, a conventional trench power transistor having improved high frequency switching and breakdown characteristics, as disclosed in U.S. Pat. No. 5,998,833, is illustrated. Each unit cell 100 of the trench power transistor includes a first-type doped drain layer 114 that has a high doping concentration (e.g. N+ doping), a first-type doped drift layer 112, a second-type doped (e.g. P-type doped) base layer 116 that is formed on the drift layer 112, a source layer 118 that has a high doping concentration, a source electrode 128b and a drain electrode 130 that are in ohmic contact respectively with the source layer 118 and the drain layer 114, and a trench electrode structure. The trench electrode structure has a trench that is defined by two opposing sidewalls 120a and a bottom 120b. The trench electrode structure includes a gate electrode 127, a trench-based source electrode 128a, and an oxide insulating layer 125 that has an insulating region 125a disposed between the gate electrode 127 and the trench-based source electrode 128a. Basically, the trench-based source electrode 128a is used to replace a part of the gate electrode 127 to reduce the CSD for decreasing the interference current and gate charge generated during the operation of the trench power transistor at high frequency, so that the breakdown voltage and the switching speed of the trench power transistor at high frequency can be improved without sacrificing the on-state resistance. However, the aforesaid intended effects are still unsatisfactory.